Chapter 4. General Principles of Information Security

4.1. Tips, Guides, and Tools

The following general principals provide an overview of good security practices:

encrypt all data transmitted over networks to help prevent man-in-the-middle attacks and eavesdropping. It is important to encrypt authentication information, such as passwords.
minimize the amount of software installed and running services.
use security-enhancing software and tools, for example, Security-Enhanced Linux (SELinux) for Mandatory Access Control (MAC), Netfilter iptables for packet filtering (firewall), and the GNU Privacy Guard (GnuPG) for encrypting files.
if possible, run each network service on a separate system to minimize the risk of one compromised service being used to compromise other services.
maintain user accounts: create and enforce a strong password policy; delete unused user accounts.
routinely review system and application logs. By default, security-relevant system logs are written to /var/log/secure and /var/log/audit/audit.log. Note: sending logs to a dedicated log server helps prevent attackers from easily modifying local logs to avoid detection.
never log in as the root user unless absolutely necessary. It is recommended that administrators use sudo to execute commands as root when required. Users capable of running sudo are specified in /etc/sudoers. Use the visudo utility to edit /etc/sudoers.

4.1. Tips, Guides, and Tools

The United States' National Security Agency (NSA) provides hardening guides and tips for many different operating systems, to help government agencies, businesses, and individuals secure their systems against attack. The following guides (in PDF format) provide guidance for Red Hat Enterprise Linux 5:

The Defense Information Systems Agency (DISA) provides documentation, checklists, and tests to help secure your system (Information Assurance Support Environment). The UNIX SECURITY TECHNICAL IMPLEMENTATION GUIDE (PDF) is a very specific guide to UNIX security - an advanced knowledge of UNIX and Linux is recommended before reading this guide.

The DISA Unix Security Checklist provides a collection of documents and checklists, ranging from the correct ownerships and modes for system files, to patch control.